This invention relates to a disk cartridge encasing disk such as a magnetic disk, an optical disk, etc., and more particularly to an improvement in the material of a hub to be fixed at the center of the disk to rotate the disk.
One prior art document relating to such a hub is Japanese Patent Application No. 60-27547 filed by the Applicant, where the hub, as shown in FIG. 9, comprises a control ring 19 made from a plastic for defining the free movement of a disk 2 in a case 1, and a flange 18 made from a thin magnetic stainless steel sheet for being chucked by magnetic attraction, the control ring 19 and the flange 18 being integrally built up by outsert molding. The control ring 18 has, on its inside surface, a positioning surface for centering with respect to the driving shaft S for a disk drive, and the flange 18 has, on its plate surface, a driving hole 23 which is engaged with a driving pin P on the driving shaft. The flange 18 and the control ring 19 are so integrated that the flange 18 may cover the opening at one side of the control ring 19.
In the formation of a hub by integrating the control ring and the flange made from different kinds of materials by outsert molding, as described above, there are the following problems as to the molding owing to differences in the material characteristics therebetween, particularly to a large difference in the coefficient of thermal expansion. For example, the coefficient of thermal expansion of iron is 1.2.times.10 to the minus fifth power (1.2.times.10.sup.-5), whereas that of nylon 6-6 is 10.about.15.times.10 to the minus fifth power (10.about.15.times.10.sup.-5), and thus there is a difference of about 10 times therebetween. When there is a large difference in the coefficient of thermal expansion between the control ring and the flange, the control ring has a larger contraction ratio when solidified, and thus the plate surface of the flange will undergo warping deformation in the plate thickness direction owing to the internal stress developed by the solidification. Once the flatness of the plate surface of the flange is deteriorated by the warping deformation, track deviation or disk flapping is liable to occur while the disk is used, and will be a fatal drawback for a high density recording disk.
Furthermore, when a hub obtained by the said outsert molding is applied to a disk cartridge of relatively small size, it is necessary to prepare a smaller hub to meet the size-down of a disk diameter, but there has been a restriction with respect to the mechanical strength to make the sizes of a control ring, for example, thickness, ring width, etc. smaller. That is, the disk can be made smaller in size, but it has been impossible to make the hub smaller in the size than given limit value. Furthermore, the positioning surface on the control ring is liable to undergo attrition by the driving shaft, thereby giving a poor centering. When the control ring is a plastic molded article, it is hard to obtain a positioning surface with good durability and reliability against attrition over a long time.